The invention relates generally to methods and apparatus for measuring fluid characteristics and in particular, to ultrasonic measurement methods and apparatus typically employed for measuring fluid flow characteristics.
Non-invasive, clamp-on transit time doppler or correlation ultrasonic flowmeters have been employed to measure or interrogate fluids, and in particular liquids. The clamp-on flowmeter typically employs longitudinal or shear wave piezoelectric crystals to interrogate the fluid. In many instances the interrogation signals in the fluid are narrow band oblique signals and the flowmeters use transit time measurements to determine the desired characteristics of the fluid being measured.
When it became more desirable to employ a so-called "wide beam" or "broad beam" as the interrogation source, for example a beam extended axially along a conduit such as a pipe or tube, it became customary to employ Lamb waves. Lamb waves in the pipe wall provide an improvement over longitudinal or shear wave interrogation sources in the clamp-on interrogation system because their broad beam axial extent makes it possible to position more easily a pair of communicating transducers. This flexible spacing, unavailable in narrow beam shear wave or longitudinal wave systems, results from a spacing which is relatively independent of the speed of sound in the fluid. Lamb waves, however, have a disadvantage in that they are dispersive. Therefore, when Lamb waves are used, the optimum frequency must be calculated as a function of pipe thickness and composition. Unfortunately, the pipe thickness and composition are not always known precisely at either or both of the transducer locations.
It is also well known that one of the advantages of using a shear wave in the pipe wall over a longitudinal wave is the lower velocity of the shear wave. And further, it is recognized that a surface wave, such as a Rayleigh wave, has a yet lower phase velocity, typically about ten percent lower than a shear wave in a given pipe material. The surface wave is therefore better suited for oblique interrogation of fluids because of the more favorable refraction angles which can be achieved. Unfortunately, however, the Rayleigh wave is also well known to attenuate significantly as the wave "penetrates" into the solid. Thus, if the thickness of a plate, w, is more than one Rayleigh wavelength in thickness (the wavelength corresponding to the wavelength of the Rayleigh wave in the solid), a significant attenuation occurs in the plate. The attenuation increases exponentially as the depth into the solid increases. Accordingly, Rayleigh and Rayleigh-like waves have never been considered practical in connection with clamp-on transducers since the signal which transfers to the fluid has always been thought to be substantially insignificant in strength when compared to the noise in the system. The present invention overcomes the attenuation restriction by means of a synthetic aperture technique.
It is therefore an object of the present invention to provide a fluid measurement apparatus and method employing Rayleigh and Rayleigh-like waves for determining one or more characteristics of the fluid. Other objects of the invention are the designation of the limitations upon the structure of the Rayleigh wave generating apparatus and method in order to effectively measure characteristics of a fluid; the use of Rayleigh-like waves for determining characteristics of a fluid; and the provision of a reliable, sensitive flowmeter detection apparatus and method. Yet further objects of the invention are a low cost, clamp-on ultrasonic transducer system, a liquid level measuring system, and a reliable and inexpensive fluid interrogation system.